Tunable laser assembly and method of control

ABSTRACT

A tunable laser assembly housed in a single enclosure and a method of control is described wherein the tunable laser, pump and semiconductor optical amplifier do not share a common optical axis but are all aligned to optical waveguides on an intervening planar lightwave circuit (PLC). Wavelength monitoring circuitry is included on the PLC to enable monitoring and control of the tunable laser center wavelength and optical bandwidth. The design of the PLC does not introduce perturbations into the swept-source laser output spectrum that would cause artifacts in imaging applications such as optical coherence tomography (OCT).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/987,102 filed on Mar. 9, 2020, and U.S. ProvisionalPatent Application No. 62/989,007 filed on Mar. 13, 2020. Thedisclosures of U.S. Provisional Patent Application 62/987,102 and U.S.Provisional Patent Application No. 62/989,007 are hereby incorporated byreference.

FIELD OF THE INVENTION

This invention relates generally to semiconductor lasers, and moreparticularly to tunable semiconductor lasers.

BACKGROUND

Tunable lasers are critical components in many optical imaging andoptical sensing systems. High output power, broad tuning, and extremelypure and stable spectral characteristics are required forhigh-performance systems. Vertical cavity semiconductor lasers (VCSELs)have proven to be good sources for these applications due to theirsingle-frequency, mode-hop-free tuning characteristics which provide forlong coherence length laser output. VCSELs differentiate themselves fromother types of tunable semiconductor lasers in that the cavity length ofthe VCSEL is short enough that only one longitudinal mode under the gaincurve is available for lasing. This is in comparison with in-plane,edge-emitting tunable lasers where multiple longitudinal modes existunder the gain curve requiring wavelength selective elements to selectlasing only one longitudinal mode. The use of light weightmicro-electro-mechanical system (MEMS) tuning elements enable high-sweeprates e.g., 10 kHz to 1 MHz over broad tuning ranges e.g., 10 nm to morethan 100 nm. VCSELs are also attractive because they are scalable towafer-level manufacturing and therefore lower cost. Operatingwavelengths can include a very wide range, based only upon theavailability of semiconductor gain elements and optical Bragg gratingreflectors, ranging from ultraviolet (UV) e.g., 250 nm out to manymicrons, e.g., more than 5 microns.

Implementation of the optical imaging or optical sensing system requiresaccurate knowledge of the tunable laser wavelength as it sweeps over itstuning range. Many methods have been used to characterize the tunablelaser wavelength, including multi-point calibration [1], built-inwavelength meters [2], bandpass filter arrays [3], wavelengthdiscriminator arrays [4], wavelength-to-power calibration [5], etalons[6,7], position sensitive detectors, [8], arrayed waveguide gratings [9]and a series of fiber Bragg gratings [10]. These methods acknowledgethat the wavelength tuning characteristics of the tunable laser willvary over time responding to environmental (e.g., temperature, pressure)and aging effects. There can also be short-term sweep-to-sweepvariations due to inherent electro-mechanical properties of the tuningelements. Moreover, instantaneous characterization of wavelength becomesmore difficult as the sweep rate increases. Modern optical imagingsystems, for example such as those used in optical coherence tomography(OCT), employ interferometers to measure the instantaneous laserwavelength. The output from the interferometer interfaces withhigh-speed data acquisition system enabling compensation for anyshort-term variations in the laser spectral sweep characteristics.However, it is important that the overall spectral properties of thelaser output e.g., center wavelength, optical bandwidth, and spectralshape, remain constant over the operating environment and lifetime ofthe laser.

The optical output power from single-spatial mode tunable VCSELs islimited by the small cavity size and thermal properties of thesemiconductor epitaxial layer structure. Output power in the range of 50mW or greater are required, particularly to achieve desiredsignal-to-noise sensitivity as the sweep rate increase to speeds inexcess of more than 200 kHz. High output power from the tunable laseralso reduces overall system costs by enabling lower cost components andassembly techniques downstream of the tunable laser. For these reasons,it is necessary to increase the output power from the VCSEL using anoptical amplifier.

Low cost is also an essential element to enable optical imaging andsensing applications in higher volume consumer markets such as roboticmachine vision, autonomous driving, and home health care OCTapplications.

The following novel concepts according to embodiments of the presentinvention provide a highly stable, low cost, tunable laser assemblybased on VCSELs and planar lightwave circuits (PLCs). The concepts areapplicable to both optically-pumped and electrically-pumped VCSELs

SUMMARY

To create a highly stable, low cost, tunable VCSEL laser assembly, themethod for combining the various optical elements that comprise thelaser assembly, i.e., the ‘packaging platform’ should take advantage ofphoto-lithographically defined, wafer-scale planar optical circuitsrather than traditional ‘optical bench’ packaging platforms. Thefunctions of numerous bulk optical elements (e.g., mirrors, lenses, beamsplitters/combiners) can be implemented at lower cost on wafer-scaleplanar optical circuits with the two-fold advantage of (1) higherspectral stability through implementation of more complex monitoring andcontrol circuits and (2) lower assembly cost through simplified opticalalignment. Therefore, the first concept used in the basic design of thishighly stable, low cost, tunable VCSEL laser assembly according to anembodiment is to use a wafer-scale planar optical circuit packagingplatform.

Many technologies exist for fabricating planar optical circuits, alsoreferred to as planar lightwave circuits (PLCs), includingsilica-on-silicon, silicon-on-insulator (SOI), and LPCVD siliconnitride. The choice of PLC technology depends highly on the technicalrequirements of the application (operating wavelength, optical loss,optical non-linearity) as well as the economic requirements (size, cost,production volume). SOI has attracted a lot of attention due tocompatibility with CMOS silicon wafer fabrication processes and hasenabled the rapidly commercializing field of silicon photonics. However,propagation in silicon waveguides is limited to wavelengths greater thanapproximately 1.1 μm. Therefore, SOI is not appropriate for typicalbio-science or life-science applications in the visible to near-infraredwavelength range (0.4-1.1 μm).

A distinguishing feature of any PLC technology is the amount of lateralwaveguide confinement that can be achieved which is related to therefractive index difference between the core and cladding. The so-called‘Δn’ or index contrast is defined as Δn=(n_(core)−n_(dad))/n_(dad) wheren_(core) and n_(dad) are the index of the waveguide core material andcladding material, respectively [11]. The higher the index contrast, thesmaller the radius of curvature that is possible for waveguide bendswhich enables smaller chips or a higher density of optical functions.The disadvantage of higher index contrast is that the bi-refringence ofthe waveguide increases. Low bi-refringence PLCs can be designed on lowindex contrast technologies such as silica-on-silicon or ion exchangeglass waveguides that have essentially no difference in the propagationcharacteristics the orthogonal of TE and TM modes, similar tosingle-mode optical fiber. High index contrast technologies such as SOIand LPCVD silicon nitride, however, can have extremely differentpropagation characteristics between TE and TM modes such that the PLC isessentially a single-polarization component. The index contrast of theselected PLC technology must be matched to the polarization requirementsof the application.

It should be noted that most PLC technologies incorporate a method toactively adjust the characteristics of the various circuit components inorder to compensate for fabrication tolerances and in some instances toimplement broad wavelength tuning. This adjustment is typicallyaccomplished with on-chip micro-heaters that provide localized heatingof the waveguide and thereby change the waveguide index of refraction.The use of micro-heaters is not fundamental to this invention, but boththe presence of micro-heaters on the PLC and the ability to adjust/tunecircuit components is assumed.

One embodiment of the present invention provides a tunable laserassembly housed in a single enclosure wherein the MEMS-VCSEL chip, pumpchip and semiconductor optical amplifier chip are not aligned to eachother (do not share a common free-space optical axis) but are allaligned to optical waveguides on an intervening planar lightwave circuit(PLC) chip.

One embodiment of the present invention provides a method forcontrolling the absolute wavelength and the optical bandwidth of aswept-source tunable laser that uses the timing information from asignal generated by a reference wavelength filter and an optical elementthat generates signal pulses corresponding to nearly equally spacedwavenumbers.

One embodiment of the present invention provides a stabilized laserincluding: a tunable semiconductor laser emitting tunable laserradiation; at least one photodetector; at least one reference wavelengthfilter; at least one optical element that generates signal pulsescorresponding to nearly equally spaced wavenumbers; and a closed loopcontroller; wherein timing information from a signal generated by thatat least one reference wavelength filter and the at least one an opticalelement that generates signal pulses corresponding to nearly equallyspaced wavenumbers are input to the closed-loop controller and theclosed-loop controller stabilizes the absolute wavelength and opticalbandwidth of said tunable laser radiation.

One embodiment of the present invention provides a swept source opticalcoherence tomography system including: a tunable semiconductor laseremitting tunable laser radiation; at least one photodetector; at leastone reference wavelength filter; at least one optical element thatgenerates signal pulses corresponding to nearly equally spacedwavenumbers; and a closed loop controller; wherein timing informationfrom a signal generated by that at least one reference wavelength filterand the at least one an optical element that generates signal pulsescorresponding to nearly equally spaced wavenumbers are input to theclosed-loop controller and the closed-loop controller stabilizes theabsolute wavelength and optical bandwidth of said tunable laserradiation; an OCT interferometer; and an OCT detector; wherein at leasta portion of the said tunable laser radiation is directed to the OCTinterferometer and the output of the OCT interferometer directed to anOCT detector for generating OCT interferograms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram of highly stable, low cost, tunable laserassembly design according to an embodiment of the present invention.

FIG. 2 shows a tunable laser assembly design according to anotherembodiment of the present invention.

FIG. 3 shows a wavelength monitoring circuit design according to anembodiment of the present invention.

FIG. 4 shows a wavelength monitoring circuit according to anotherembodiment of the present invention.

FIG. 5 a wavelength monitoring circuit to another embodiment of thepresent invention.

FIG. 6 shows a tunable laser assembly design according to anotherembodiment of the present invention.

FIG. 7 shows a tunable laser assembly design according to anotherembodiment of the present invention.

FIG. 8 shows a tunable laser assembly design according to anotherembodiment of the present invention.

FIG. 9 shows a tunable laser assembly design according to anotherembodiment of the present invention.

FIG. 10 shows an assembly method for integrating photodetectors in thetunable laser assembly according to an embodiment of the presentinvention.

FIG. 11 shows an assembly method for integrating photodetectors in thetunable laser assembly according to another embodiment of the presentinvention.

FIG. 12 shows an assembly method for integrating a MEMS-tunable VCSEL inthe tunable laser assembly according to an embodiment of the presentinvention.

FIG. 13 shows an assembly method for integrating a MEMS-tunable VCSEL inthe tunable laser assembly according to another embodiment of thepresent invention.

FIG. 14 shows an assembly method for heterogeneous integration of thepump laser and semiconductor optical in the tunable laser assemblyaccording to another embodiment of the present invention.

FIG. 15 shows an electrically-pumped VCSEL tunable laser assembly designaccording to another embodiment of the present invention.

FIG. 16 shows the influence of changes in the tunable laser sweepvelocity on the optical bandwidth.

FIG. 17A shows a method of controlling the tunable laser sweep bandwidthand tuning trajectory by monitoring the timing of optical etalon pulsesrelative a timer starting point generated by a reference-wavelengthfilter. FIG. 17B is a schematic diagram of a highly stable tunable laserwith a closed-loop controller according to an embodiment of the presentinvention.

FIG. 18 shows the design of a PLC wavelength monitoring circuitaccording to another embodiment of the present invention that utilizes amicro-ring resonator to generate the optical etalon pulses and a 4-stagecascaded MZI filter to generate the reference-wavelength filter signal.

FIGS. 19A and 19B show experiments data measured for the PLC designdescribed in FIG. 18.

FIG. 20 shows the design of a PLC wavelength monitoring circuitaccording to another embodiment of the present invention that utilizes asingle micro-ring resonator to generate the optical etalon pulses and a3-stage coupled micro-ring resonator to generate thereference-wavelength filter.

FIG. 21A shows the designed optical signal from the MRR1 thru port, andFIG. 21B shows the signal in the MRR3 drop port.

FIG. 22 shows an optical circuit using a multi-channel fiber Bragggrating to control the center wavelength and optical bandwidth of atunable laser.

FIG. 23 shows the design of a single multi-channel fiber Bragg gratingthat generates both the optical etalon pulses and thereference-wavelength filter.

FIGS. 24A and 24B show the operation of a tunable MEMS-VCSEL without andwith optical bandwidth control respectively.

FIGS. 25A and 25B are schematic diagrams of example swept source OCTsystems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description of illustrative embodiments according to principles ofthe present invention is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments of the inventiondisclosed herein, any reference to direction or orientation is merelyintended for convenience of description and is not intended in any wayto limit the scope of the present invention. Relative terms such as“lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,”“down,” “top” and “bottom” as well as derivative thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) should be construed torefer to the orientation as then described or as shown in the drawingunder discussion. These relative terms are for convenience ofdescription only and do not require that the apparatus be constructed oroperated in a particular orientation unless explicitly indicated assuch. Terms such as “attached,” “affixed,” “connected,” “coupled,”“interconnected,” and similar refer to a relationship wherein structuresare secured or attached to one another either directly or indirectlythrough intervening structures, as well as both movable or rigidattachments or relationships, unless expressly described otherwise.Moreover, the features and benefits of the invention are illustrated byreference to the exemplified embodiments. Accordingly, the inventionexpressly should not be limited to such exemplary embodimentsillustrating some possible non-limiting combination of features that mayexist alone or in other combinations of features; the scope of theinvention being defined by the claims appended hereto.

This disclosure describes the best mode or modes of practicing theinvention as presently contemplated. This description is not intended tobe understood in a limiting sense, but provides an example of theinvention presented solely for illustrative purposes by reference to theaccompanying drawings to advise one of ordinary skill in the art of theadvantages and construction of the invention. Although some elementsdisclosed herein are implemented on a chip or chipset without loss ofgenerality, it is understood that many of these elements may also beimplemented, for example, on one or more chips and/or one or moreoptical elements. In the various views of the drawings, like referencecharacters designate like or similar parts.

FIG. 1 is a schematic of a tunable laser assembly incorporating aMEMS-VCSEL chip, pump laser chip, semiconductor amplifier chip andplanar lightwave circuit (PLC) chip all mounted to a common baseplate220. A TEC 210 under the baseplate on which the semiconductor componentsare placed controls the temperature. As shown in FIG. 1, a tunable laserassembly according to an embodiment of the present invention includes anenclosure 200 containing a MEMS-VCSEL chip 100 and a PLC chip 120. Thelaser radiation from the MEMS-VCSEL is coupled to waveguide 302 on thePLC. The laser radiation from the pump chip 110 is coupled to thepump-input waveguide 301 on the PLC and propagates to the MEMS-VCSELchip via the wavelength division multiplexer (WDM) circuit 300 on thePLC. For efficient operation, the WDM circuit is designed so that amajority of the pump laser radiation propagates to the MEMS-VCSEL chip100 and a smaller amount propagates via waveguide 303 to the pumpmonitor photodetector 140. The laser radiation from the optically-pumpedMEMS-VCSEL chip in waveguide 302 propagates to the WDM circuit outputwaveguide 307 and further propagates to the PLC output waveguide 304 viaoptical power splitter circuit 310. For efficient operation, the WDMcircuit 300 is designed so that the majority of the MEMS-VCSEL chiplaser radiation in waveguide 302 propagates to the WDM circuit outputwaveguide 307 and a smaller amount propagates to the pump-inputwaveguide 301. The optical splitter circuit 310 is designed so that themajority of the MEMS-VCSEL laser radiation propagates to the PLC outputwaveguide 304 and a smaller amount propagates via waveguide 308 to thewavelength monitoring circuit (WMC) 320. The output laser radiation fromthe WMC propagates in PLC output waveguides 305 and 306 where the laserradiation is monitored by wavelength monitor photodetectors 151 and 152mounted on a common substrate 150. The laser radiation from the PLCoutput waveguide 304 is coupled to the semiconductor optical amplifierchip 130 where the laser radiation is amplified and then propagatesthrough optical lens 160, through the window in the enclosure wall 240and is coupled into the optical output fiber 230.

For use in demanding imaging applications like OCT, the pump laser chip110 must be a low-noise, single-frequency laser such a distributedfeedback (DFB), distributed Bragg reflector (DBR), volume holographicgrating stabilized (VHG), or other external cavity laser.Single-frequency lasers have lower relative-intensity noise (RIN)compared to multi-mode lasers. The pump laser RIN is transferred to theMEMS-VCSEL laser output, so it is important for the pump RIN to be aslow as possible, preferably below −135 dB/Hz with a side-modesuppression ratio (SMSR) of 30 dB or greater. For less demanding sensingapplications than OCT, it is possible that higher RIN multi-longitudinalmode Fabry-Perot laser pump chips can be used.

There are many different possible methods for implementing the WMC 320.The embodiment shown in FIG. 2 consists of a single micro-ring resonator(MRR) 400 with the output signals on waveguides 305 and 306corresponding to the ‘through’ and ‘drop’ ports of the MRR,respectively. The term ring resonator is used in this application toindicate any looped resonator design. When the shape is elongated with astraight section, the term ‘racetrack’ resonator is also used [17]. TheMRR in FIG. 2 is shown as a circular loop, but it can also beimplemented as a racetrack, or with more complex multi-ring coupled MRRconfigurations.

An alternative WMC embodiment, shown in FIG. 3, consists of a MRRfollowed by an optical splitter 430 and MZI filter 440, both of whichare on the ‘drop’ port of the MRR. The output signal on waveguides 305corresponds to the ‘drop’ port of the MRR. The wavelength spacingbetween the optical pulses on waveguide 305 is determined by thefree-spectral range (FSR) of the MRR, which can range from the order of˜1 pm to ˜100 nm depending on the radius of the MRR which is determinedby the particular PLC implementation technology. For the application ofmonitoring/controlling the bandwidth of the tunable laser, the FSR ofthe MRR is desirable to be in the range of 0.1 nm to 10 nm. The outputsignal on waveguide 306 corresponds to the ‘drop’ port that has beenfurther filtered by the MZI filter 440 resulting in a reduced the numberof optical output signals pulses generated as the laser sweeps over itswavelength range. The FSR of the MZI filter, for example, could bedesigned to be 4 times larger than the FSR of the MRR so that only every4^(th) pulse is transmitted. Multiple MZI filters can also be cascadedto generate more complex filter functions. For the application ofmonitoring/controlling the center wavelength of the tunable laser, adesirable number of optical pulses is in the range of 1 to 10 pulses asthe tunable laser sweeps over its wavelength range.

Another embodiment of the WMC, shown in FIG. 4, consists of a MRR 400and an on-chip Bragg grating 470. The input signal to the WMC on inputwaveguide 308 is divided by optical splitter 450, with some of theoptical power is directed to directional coupler 460 and the remainingpower is directed to the ‘through’ port of the MRR 400. One of theoutputs of the directional coupler 460 is connected to the Bragg grating470. The optical signal reflected by the grating propagates to theunused directional coupler input port 490 and becomes the signal in theoutput waveguide 305. The Bragg grating can be designed to reflect onlyone wavelength across the tuning range of the tunable laser. For theapplication of monitoring/controlling the center wavelength of thetunable laser, it is desirable to have the single wavelength pulselocated within +/−20 nm of the center wavelength of the tunable laser.The signal in the MRR ‘drop’ port waveguide 420 becomes the signal outthe output waveguide 306 and can be used for monitoring/controlling thebandwidth of the tunable laser.

Those skilled in the art will recognize that there are many methods bywhich the laser radiation to/from the semiconductor components (i.e.,MEMS-VCSEL, pump, SOA, photodetectors) can be coupled to/from the PLC.The embodiment shown in FIG. 1 indicates direct butt-coupling betweenthe semiconductor components and the PLC optical waveguides. In a secondembodiment of the present invention, shown in FIG. 5, optical lenses170, 185, and 165 couple the MEMS-VCSEL, pump, and SOA to the PLC,respectively. One skilled in the art will also recognize that thefunction of a single lens can also be implemented with more complexmulti-element lens configurations. One of the benefits of using a lensto couple the MEMS-VCSEL to the PLC waveguide is that a quarter-wave(λ/4) polarization waveplate 180 can be inserted between the MEMS-VCSEL100 and the MEMS-VCSEL input waveguide 302. Orienting the polarizationaxis of the λ/4-waveplate at 45 degrees to the preferred polarizationaxis of the PLC prevents the pump light that is reflected from theMEMS-VCSEL from propagating back to the pump laser chip because thereflected pump light is orthogonal to the preferred polarization of thePLC and is therefore highly attenuated. Similarly, this configuration ofthe λ/4-waveplate in combination with the preferred polarization axis ofthe PLC prevents backwards propagating laser radiation from the SOA(e.g., amplified spontaneous emission (ASE) and/or amplified reflectedsignal) that reflects from the MEMS-VCSEL from propagating forward inthe PLC and setting up a potential laser cavity between the SOA and theMEMS-VCSEL.

In another embodiment of the present invention, shown in FIG. 6, anexternal cavity reflector (ECR) circuit 330 is implemented on the PLC toform an external cavity laser in combination with the pump chip 110. Inthis case, the pump chip would typically have an anti-reflection coatingthe facet facing the PLC. Narrow-band reflection from the ECR circuitcreates and a narrow-linewidth, single-frequency laser for low-noisepumping of the MEMS-VCSEL. Those skilled in the art will recognize thatthere are many ECR circuits, for example Bragg gratings [12] ormicro-ring resonators [13] that can be implemented on PLCs to create anexternal cavity laser.

In another embodiment of the present invention, shown in FIG. 7, anoptical isolator 190 is placed between the pump laser chip 110 and thepump-input waveguide 301. This isolator prevents pump light that isreflected from the MEMS-VCSEL from propagating back to the pump laserchip. An output isolator 195 is also placed between the SOA 130 and theoptical fiber 230 to prevent reflections from propagating back to theSOA.

In another embodiment of the present invention, shown in FIG. 8, andoptical isolator 196 is placed between the PLC output waveguide 304 andthe SOA. This isolator prevents backwards propagating laser radiationfrom the SOA that reflects from the MEMS-VCSEL from setting up apotential laser cavity between the SOA and the MEMS-VCSEL.

Those skilled in the art will recognize that angled interfaces are oftenused in order to reduce reflections between optical components. Inanother embodiment of the present invention, shown in FIG. 9, the inputand output PLC waveguides (301, 302, 303, 304, 305, 306) intersect thechip facet at an angle, as does the waveguide on the SOA 131.

In another embodiment of the present invention, shown in FIG. 10, thephotodetectors are attached directly to the PLC. The pump monitorphotodiode 140 is mounted on a substrate 141 which is attached to theside of the PLC using glue 700 or similar attachment material.Similarly, wavelength monitor photodetectors 151 and 152 are mounted ona common substrate 150 which is attached the side of the PLC using glue700 or similar attachment material.

In another embodiment of the present invention the photodetectors areattached directly to the PLC via flip-chip integration. The photodiodesubstrates 141 and 150 are mounted on top surface of the PLC, as shownin the top view in FIG. 11, and attached using solder 701 or otherattachment material as shown in the side view in FIG. 8. Light in theoptical waveguide 306 is directed on to the photodetector 152 by aturning mirror 450 or other method of reflecting the beam upwardsapproximately normal to the PLC top surface. A surface grating, forexample, is another method to reflect the optical beam from thewaveguide up to the photodetector.

In another embodiment of the present invention the MEMS-VCSEL isattached directly to the PLC via flip-chip integration. The MEMS-VCSEL100 is mounted on top surface of the PLC, as shown in the top view inFIG. 12, and attached using solder 701 or other attachment material asshown in the side view in FIG. 12. Light from the MEMS-VCSEL is directedinto the optical waveguide 302 by a turning mirror 450 or other methodof coupling into the waveguide. A surface grating, for example, isanother method to couple the optical beam from the MEMS-VCSEL into thewaveguide.

In another embodiment of the present invention the optical signal fromMEMS-VCSEL coupled to the PLC via an external turning mirror thatenables vertical alignment of the optical signal to the PLC waveguide bylateral adjustment of the turning mirror. The MEMS-VCSEL 100 is mountedon a common substrate 470 with the PLC chip 120, as shown in the topview in FIG. 13. The vertical (i.e., normal to the PLC waveguidepropagation plane) surface emission of MEMS-VCSEL is converted into thehorizontal (i.e., parallel to the PLC waveguide propagation plane)direction via the turning mirror 460. Precise vertical alignment (z-axisdirection) of the MEMS-VCSEL optical beam to the PLC input waveguide 320can be accomplished by adjustment of the turning mirror along the x-axisdirection. This skilled in the art will recognize that the reflectingsurface of the turning mirror 460 can be designed to act as a lens tofocus the light into the PLC input waveguide 320, or that a bulk lenscan be placed in between the MEMS-VCSEL and the PLC input waveguide toimprove the coupling efficiency.

In another embodiment of the present invention the pump laser and SOAare hybrid or heterogeneously integrated on the PLC. The pump laser 110and SOA 130 are mounted on the top surface of the PLC, as shown in thetop view in FIG. 13 and bonded to the PLC as shown in the side view inFIG. 14. There are various techniques for integration of the III-Vmaterials that comprise the pump and SOA active regions with the PLC. Inrecent years, ‘heterogenous’ is more commonly used to describe bondingof III-V chips on a wafer with coarse alignment and subsequentlyprocessed at the wafer level, ‘hybrid’ is more commonly used forsoldering or bonding individual functional die on a common substrate[14]. Heterogeneous integration enables high-performance,narrow-linewidth extended cavity DFB, DBR [15] or micro-ring resonatorlasers by coupling the active region of the pump laser 110 to the ECRcircuit 330, which in FIG. 14 is shown as a Bragg grating.

Whereas all the previous embodiments have considered an optically-pumpedMEMS-VCSEL, the present invention is also applicable to anelectrically-pumped MEMS-VCSEL. One embodiment of an electrically-pumpedtunable MEMS-VCSEL according the present invention is shown in FIG. 15.In the case of the electrically pumped MEMS-VCSEL, the WDM component isreplaced with an optical splitter 500. The laser radiation from theelectrically-pumped MEMS-VCSEL chip 600 in waveguide 502 propagates tothe optical splitter 500 and further propagates to the PLC outputwaveguide 504 via optical power splitter circuit 510. A small sample ofthe laser radiation propagates to the output power monitor photodetector140 via waveguide 503. The optical splitter circuit 510 is designed sothat the majority of the MEMS-VCSEL laser radiation propagates to thePLC output waveguide 504 and a smaller amount propagates via waveguide508 to the wavelength monitoring circuit (WMC) 520. The output laserradiation from the WMC propagates in PLC output waveguides 505 and 506where the laser radiation is monitored by wavelength monitorphotodetectors 151 and 152 mounted on a common substrate 150. The laserradiation from the PLC output waveguide 504 is coupled to thesemiconductor optical amplifier chip 130 where the laser radiation isamplified and then propagates through optical lens 160, through thewindow in the enclosure wall 240 and is coupled into the optical outputfiber 230.

All previous embodiments described for the optically pumped MEMS-VCSELapply to the electrically-pumped MEMS-VCSEL. Namely, the WMC 520 has theembodiments described for WMC 320 (FIGS. 2-4). The use of butt coupling(FIG. 1), lens coupling (FIG. 5), quarter-wave polarization plate toprovide optical isolation for the MEM-VCSEL (FIG. 5), angled waveguides(FIG. 9), hybrid and heterogeneous integration (FIGS. 10-14) are allembodiments the can be implemented with an electrically-pumpedMEMS-VCSEL.

There are several possible methods to use the signals generated by theWMC 320 to control the absolute wavelength (center wavelength) andtuning bandwidth, respectively. Open loop operation of a MEMS-VCSELswept laser sources presents many challenges in maintaining a stableoutput over long operating time frames and/or changing environmentalconditions. Long term charging effects in the MEMS structure lead tochanges in the effective voltage that is applied to the device. As theMEMS structure is an electrostatically-controlled moving membrane, therelationship between the voltage on the electrodes and the mirrorposition is highly non-linear. Slight changes in operating DC level canresult in large changes to the sweep profile and ultimately the overallbandwidth that is contained within a given time window. Additionally,the mechanical damping of the device is highly sensitive to thesurrounding environment. Open-loop calibration/corrections can beapplied, but these require extensive production characterizationprocedures and long-term testing.

To enable robust and long-term operation it is desired that an opticalreference signal be used to monitor and subsequently control the highvoltage drive signals to the tunable MEMS-element such that the sweptbandwidth (the ‘optical bandwidth’, or ‘bandwidth’) is maintained underall operating conditions and timeframes. This optical signal is used togenerate timing information which has a direct correlation to thebandwidth and overall sweep trajectory. The typical mechanism forbandwidth loss or gain is mainly that the sweep velocity changes, asillustrated in FIG. 16. As the defined active sweep period is typicallyfixed by other system parameters, any shift in the sweep velocitydirectly results in a change in the overall sweep bandwidth within agiven time window. Under this assumption, it can be shown thatcontrolling the time difference (t2−t1 in FIG. 16) required to sweep adefined bandwidth (lambda2−lambda1 in FIG. 16) is sufficient tomaintaining the overall bandwidth of the tunable swept-sourceMEMS-VCSEL.

An optical etalon can be used to generate electrical pulses (via zerocrossing detection) each of which correspond to nearly equally spacedwavenumbers. An electronic counter circuit can then be used to generatea measure of the time (deltaT) required for the device to move from astarting wavenumber to an ending wavenumber, as shown in FIG. 17A. Thecounter electronics combined with the etalon allow a very fineresolution and adjustable timing “marker” to be placed at an ideallocation within the sweep trajectory by selecting a programmable ‘Nth’pulse as the control marker. This implementation for marking the sweepend-point is highly advantageous compared to using fixed-wavelengthreference component, such a fiber Bragg grating (FBG), notch filter, orbandpass filter, which are not flexible enough to select a properend-point for all MEMS-VCSEL devices, due to variation in absolute sweepwavelength, difference in sweep rates, and/or different bandwidthrequirements. For ensuring absolute wavelength accuracy, the timerstarting point (t1, lambda1) is generated by the Reference λ-Filterwhile the second timing marker (t2) is generated by the etalon,selecting the appropriate ‘Nth’ pulse to use for bandwidth control. ThedeltaT generated by the counter electronics 1320 may be used as feedbackin a proportional-integral-derivative control algorithm (PID) toimplement a closed-loop controller 1300, as shown in FIG. 17B. The PID1310 measures the difference between the deltaT and a reference timecorresponding to the desired bandwidth (refT). Tuning coefficients areapplied to this difference generating a PID output that adjusts the gain1340 of the high voltage signal 1330 driving the MEMS-VCSEL 1360 in thetunable laser assembly 1350. This minimizes the difference betweendeltaT and refT, maintaining the desired optical bandwidth.

One embodiment to implement the center wavelength and optical bandwidthcontrol method described in the previous section is the integrated WMCcircuit shown in FIG. 4. In this example, the timer starting point (t1,lambda1) is generated by the integrated Bragg grating Reference λ-Filter470 while the second timing marker (t2) is generated by the etalonpulses from the MRR 400, selecting the appropriate ‘Nth’ pulse to usefor bandwidth control. The MRR serves as the optical etalon, providingoptical pulses in the MRR ‘drop’ port waveguide 420 in similar manner tothe transmitted signal through a bulk optical etalon. Resonances occurin the transmission spectrum of the MRR when the perimeter of the ring,L, is an integral number of wavelengths (L=2πr for a circular ring ofradius r). The resonances are spaced by FSR_(MRR)=c/n_(g)L, where n_(g)is the group index of the waveguide, n_(g)=n_(eff)+f_(o)(dn_(eff)/df),with f_(o)=optical frequency=c/λ_(o) where λ_(o) is the vacuumwavelength [16].

Another embodiment to implement the center wavelength and opticalbandwidth control method described previously is the integrated opticalcircuit PLC chip shown in FIG. 18. The optical signal from the tunablelaser enters the PLC on the input waveguide 810. The majority of thelaser input signal is directed by the optical directional coupler 830through a sequence of spiral waveguides 850 and exits the chip via theoutput waveguide 820. The spiral waveguides have a radius of curvaturedesigned to have low loss for the fundamental propagating waveguide mode(i.e., TE mode) and to have high loss for radiation propagating in apolarization orthogonal (i.e., TM mode) to the fundamental propagatingpolarization mode. The purpose of the spiral waveguides is to strip offany of the unwanted radiation propagating in the polarization orthogonalto the fundamental polarization mode and prevent cross-coupling betweenthe two polarization modes that would cause an artifact in the OCTimage. A smaller portion of the laser input signal is directed bydirectional couplers 830 and 840 to the integrated MRR 860. The MRRserves as the optical etalon, providing optical pulses in the MRR ‘thru’port waveguide 870 in similar manner to the transmitted signal through abulk optical etalon. The measured optical signal from the MRR thru portis shown in FIG. 19A. The MRR in this particular implementation wasdesigned to have an FSR of approximately 0.6 nm. The reciprocal opticalpulses in the MRR ‘drop’ port waveguide 880 is transmitted through a4-stage MZI filter 890 that is designed to block the transmission of theMRR optical pulses except those spaced approximately 10 nm apart. Themeasured optical power at the output of the 4-stage MZI filter is shownin FIG. 19B. In this embodiment, the timer starting point (t1, lambda1)is generated by selecting one of the pulses from the output of the4-stage MZI filter while the second timing marker (t2) is generated bythe etalon pulses from the MRR thru port, selecting the appropriate‘Nth’ pulse to use for bandwidth control. The pulse from the 4-stage MZIfilter used as the timer starting point can also be used for centerwavelength control.

Another embodiment to implement the center wavelength and opticalbandwidth control method described previously is the 3-stage MRRintegrated optical circuit shown in FIG. 20. The optical signal from thetunable laser enters the circuit on the input waveguide 900 and isincident on the first MRR (MRR1). MRR1 serves as the optical etalon,providing optical pulses in the MRR1 ‘thru’ port waveguide 910 insimilar manner to the transmitted signal through a bulk optical etalon.The designed optical signal from the MRR thru port is shown in FIG. 21Aand has an FSR of approximately 0.55 nm. The signal in the MRR1 dropport is coupled to the 2^(nd) MRR (MRR2) and in turn the signal in theMRR2 drop port is coupled to the 3^(rd) MRR (MRR3). MRR2 and MRR3 aredesigned in a vernier way so that only one pulse from MRR1 istransmitted. The signal in the MRR3 drop port waveguide 920 provides theReference Wavelength Filter (λ-Filter), as shown in FIG. 21B, there isonly one transmitted pulse over wavelength range 1000 nm-1120 nm. Inthis embodiment, the timer starting point (t1, lambda1) is generated bythe signal in the MRR3 drop port waveguide while the second timingmarker (t2) is generated by the etalon pulses from the MRR1 thru port,selecting the appropriate ‘Nth’ pulse to use for bandwidth control. Thepulse from the MRR3 drop port used as the timer starting point can alsobe used for center wavelength control. This embodiment has the advantagecompared to the previous embodiment with 4-stage MZI filter in thatthere is only one signal pulse out of the Reference λ-Filter over theentire tuning range, which makes establishing the control loop easierthan if there are multiple signals.

This same method can be applied for use in any type of tunable sweptsource laser and is not limited to the integrated optical assemblyembodiments in this disclosure. For example, an FBG can be used as theReference λ-Filter and a Mach-Zehnder Interferometer (MZI) or FBG havingmultiple reflection peaks may be used in a similar manner as the etalonto obtain the same timing information for the follow-on controlalgorithms. An embodiment using a specially-designed FBG with multiplereflection peaks is shown in FIG. 22. The output from the tunable laseris incident on an FBG that is designed to have one main reflection peakthat is higher in amplitude than all the other reflection peaks, asshown in FIG. 23. There are no reflection peaks for wavelengths longerthan the main reflection peak (which is at 1300 nm in this example), andthere is a guard band of approximately 10 nm on the short wavelengthside of the main reflection peak where there are also no reflectionpeaks. For wavelengths shorter than approximately 10 nm below the mainpeak there is a series of reflection peaks spaced equally in thefrequency domain. Two different designs are shown in FIG. 23, one designhas the side peaks having approximately 1 nm spacing and the otherhaving approximately 0.5 nm spacing. The reflectance of the main peak isgreater than 90%, whereas the reflectance of the side peaks isapproximately 40%. This difference in reflection amplitude makes iteasier to distinguish between the main peak and the side peaks and toset appropriate thresholds for detecting the main peak, which serves asthe Reference λ-Filter, compared to detecting the side peaks which arethe etalon signals. In this example, the timer starting point (t1,lambda1) is generated by the main reflectivity peak while the secondtiming marker (t2) is generated by the etalon pulses from the sidepeaks, selecting the appropriate ‘Nth’ pulse to use for bandwidthcontrol.

The application of the optical bandwidth control method described in thepreceding sections is demonstrated in FIGS. 24A and 24B, which showoperation of a MEMS_VCSEL without and with bandwidth control. Withoutbandwidth control, there is a change in bandwidth of approximately −5%over 20 hours as the DC bias operating point slowly drifts (FIG. 24A).This change in bandwidth is eliminated with bandwidth control methodengaged (FIG. 24B).

Optical Coherence Tomography (OCT) is a non-invasive, interferometricoptical imaging technique that can generate micron resolution 2D and 3Dimages of tissue and other scattering or reflective materials. Withapplications in medicine, biological research, industrial inspection,metrology, and quality assurance, OCT can be used for subsurfaceimaging, surface profiling, motion characterization, fluid flowcharacterization, index of refraction measurement, birefringencecharacterization, scattering characterization, distance measurement, andmeasurement of dynamic processes. The most common implementation of OCTis spectral/Fourier domain OCT (SD-OCT), which uses a broadband lightsource, interferometer, and spectrometer. An alternate implementation ofOCT is swept source OCT (SS-OCT). SS-OCT uses a tunable laser (sometimescalled a wavelength swept laser), interferometer, OCT detector, and highspeed analog to digital (A/D) converter. The tunable laser sweeps anemission wavelength in time which is used as input to an OCTinterferometer. An OCT interferogram is formed by interfering anddetecting light from a sample arm with light from a reference arm in theOCT interferometer, which is detected by the OCT detector and digitizedby the A/D converter. Processing the digitized interferogram generates areflectivity vs. depth profile of the sample, called an A-scan. MultipleA-scans can be obtained to generate two dimensional OCT images or threedimensional OCT volumes.

FIGS. 25A and 25B show schematic diagrams of example swept source OCTsystems. Depending on the wavelength of operation, a coupler based orcombined coupler and circulator based interferometer might be preferred.FIG. 25A shows a swept source OCT system 1400 in which light from atunable laser 1405 is directed to a coupler 1410 which splits lightbetween a sample path 1415 and a reference path 1420. Light from thesample path 1415 and light from the reference path 1420 are combined ata path interfering element 1425 and directed to an OCT detector 1430.The electrical signal from the OCT detector 1430 is digitized by the A/Dconverter 1435. FIG. 25B shows a swept source OCT system 1450 in whichlight from a tunable laser 1455 is directed to a coupler 1460 whichsplits light between a sample path 1465 including an optical circulator1467 and a reference path 1470 including an optical circulator 1472.Light from the sample path 1465 and light from the reference path 1470are combined at a path interfering element 1475 and directed to an OCTdetector 1480. The electrical signal from the OCT detector 1480 isdigitized by the A/D converter 1485. While FIGS. 25A and 25B show commonOCT system topologies, other OCT system topologies are possibleincluding using various combinations of couplers and optical circulatorsnot shown in FIG. 25. Components in a SS-OCT system may be any one of orany combination of fiber optic components, free space components,photonic integrated circuits (PIC), and planar lightwave circuits (PLC).

While the present invention has been described at some length and withsome particularity with respect to the several described embodiments, itis not intended that it should be limited to any such particulars orembodiments or any particular embodiment, but it is to be construed withreferences to the appended claims so as to provide the broadest possibleinterpretation of such claims in view of the prior art and, therefore,to effectively encompass the intended scope of the invention.Furthermore, the foregoing describes the invention in terms ofembodiments foreseen by the inventor for which an enabling descriptionwas available, notwithstanding that insubstantial modifications of theinvention, not presently foreseen, may nonetheless represent equivalentsthereto.

REFERENCES

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What is claimed is:
 1. A method for controlling the absolute wavelength and the optical bandwidth of a swept-source tunable laser that uses the timing information from a signal generated by a reference wavelength filter and an optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
 2. The method of claim 1, wherein the said timing information from a signal generated by a reference wavelength filter is used for absolute wavelength control and said timing information from an optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers is used for optical bandwidth control.
 3. The method of claim 2, wherein the said timing information is comprised of a timer starting point generated by said reference wavelength filter and a second timing marker generated by said optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
 4. The method of claim 3, wherein said second timing marker is generated by selecting the appropriate ‘Nth’ pulse from said optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
 5. The method of claim 1 wherein said reference wavelength filter and said optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises a fiber Bragg grating (FBG) with multiple reflection peaks.
 6. The method of claim 5, wherein said FBG with multiple reflection peaks has one main reflection peak that is higher in amplitude than all the other reflection side peaks and serves as said wavelength reference filter; all other reflection side peaks generate said signal pulses corresponding to nearly equally spaced wavenumber.
 7. The method of claim 6, wherein said main reflection peak has a reflectance of greater than 90% and said reflection side peaks have a reflectance of less than approximately 40%.
 8. The method of claim 6, wherein said main reflection peak is located within +/−20 nm of the center wavelength of the tunable laser.
 9. The method of claim 6, wherein said reflection side peaks have a free spectral range (FSR) in the range of 0.1 nm to 10 nm.
 10. The method of claim 1, wherein said optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises an etalon.
 11. The method of claim 1, wherein said reference wavelength filter comprises one of a fiber Bragg grating (FBG), notch filter, or bandpass filter,
 12. The method of claim 1, wherein said reference wavelength filter and said optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises a planar lightwave circuit (PLC) with integrated wavelength monitoring circuit (WMC).
 13. The method of claim 12, wherein said WMC comprises at least one selected from a list of: a Bragg grating, a micro-ring resonator (MRR), and a Mach-Zehnder interferometer (MZI).
 14. The method of claim 13, wherein said at least one MRR or at least one MZI generates said signal pulses corresponding to nearly equally spaced wavenumber used for optical bandwidth control.
 15. The method of claim 14, wherein said at least one MRR or at least on MZI has a FSR in the range of 0.1 to 10 nm.
 16. The method of claim 13, wherein said at least one selected from a list of: a Bragg grating, a MRR or a MZI generates signal pulses used for absolute wavelength control.
 17. The method of claim 16, wherein said at least one selected from a list of: a Bragg grating, a MRR or a MZI generates said signal pulses in the range of 1 to 10 pulses as the tunable laser sweeps over its wavelength range.
 18. The method of claim 12, wherein said WMC comprises at least one Bragg grating.
 19. The method of claim 18, wherein said Bragg grating comprises said wavelength reference filter.
 20. The method of claim 18, wherein said at least one Bragg grating has a main reflection peak located within +/−20 nm of the center wavelength of the tunable laser.
 21. A stabilized laser comprising: a tunable semiconductor laser emitting tunable laser radiation; at least one photodetector; at least one reference wavelength filter; at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers; and a closed loop controller; wherein timing information from a signal generated by that at least one reference wavelength filter and the at least one an optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers are input to the closed-loop controller and the closed-loop controller stabilizes the absolute wavelength and optical bandwidth of said tunable laser radiation.
 22. The stabilized laser of claim 21, wherein said closed-loop controller implements a proportional-integral-derivative (PID) algorithm based on said timing information.
 23. The stabilized laser of claim 21, wherein the said timing information from a signal generated by at least one reference wavelength filter is used for absolute wavelength control and said timing information from at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers is used for optical bandwidth control.
 24. The stabilized laser of claim 21, wherein the said timing information is comprised of a timer starting point generated by said at least one reference wavelength filter and a second timing marker generated by said at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
 25. The stabilized laser of claim 24, wherein said second timing marker is generated by selecting the appropriate ‘Nth’ pulse from said optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
 26. The stabilized laser of claim 21 wherein said at last one reference wavelength filter and said at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises a fiber Bragg grating (FBG) with multiple reflection peaks.
 27. The stabilized laser of claim 26, wherein said FBG with multiple reflection peaks has one main reflection peak that is higher in amplitude than all the other reflection side peaks and serves as said wavelength reference filter; all other reflection side peaks generate said signal pulses corresponding to nearly equally spaced wavenumber.
 28. The stabilized laser of claim 27, wherein said main reflection peak has a reflectance of greater than 90%.
 29. The stabilized laser of claim 27, wherein said main reflection peak is located within +/−20 nm of the center wavelength of the tunable laser.
 30. The stabilized laser of claim 27, wherein said reflection side peaks have a reflectance of less than approximately 40%.
 31. The stabilized laser of claim 27, wherein said reflection side peaks have a free spectral range (FSR) in the range of 0.1 nm to 10 nm.
 32. The stabilized laser of claim 21, wherein said at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises an etalon.
 33. The stabilized laser of claim 21, wherein said at least one reference wavelength filter comprises one of a fiber Bragg grating (FBG), notch filter, or bandpass filter,
 34. The stabilized laser of claim 21, wherein said at least one reference wavelength filter and said at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises a planar lightwave circuit (PLC) with integrated wavelength monitoring circuit (WMC).
 35. The stabilized laser of claim 34, wherein said WMC comprises at least one selected from a list of: a Bragg grating, a micro-ring resonator (MRR), and a Mach-Zehnder interferometer (MZI).
 36. The stabilized laser of claim 35, wherein said at least one MRR or at least one MZI generates said signal pulses corresponding to nearly equally spaced wavenumber used for optical bandwidth control.
 37. The stabilized laser of claim 36, wherein said at least one MRR or at least on MZI has a FSR in the range of 0.1 to 10 nm.
 38. The stabilized laser of claim 35, wherein said at least one selected from a list of: a Bragg grating, a MRR or a MZI generates signal pulses used for absolute wavelength control.
 39. The stabilized laser of claim 38, wherein said at least one selected from a list of: a Bragg grating, a MRR or a MZI generates said signal pulses in the range of 1 to 10 pulses as the tunable laser sweeps over its wavelength range.
 40. The stabilized laser of claim 34, wherein said WMC is comprised of at least one Bragg grating.
 41. The stabilized laser of claim 40, wherein said at least one Bragg grating comprises said at least one wavelength reference filter.
 42. The stabilized laser of claim 41, wherein said at least one Bragg grating has a main reflection peak located within +/−20 nm of the center wavelength of the tunable laser.
 43. The stabilized laser of claim 21, wherein said tunable semiconductor laser is a tunable MEMS-VCSEL.
 44. A swept source optical coherence tomography system comprising: a tunable semiconductor laser emitting tunable laser radiation; at least one photodetector; at least one reference wavelength filter; at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers; and a closed loop controller; wherein timing information from a signal generated by that at least one reference wavelength filter and the at least one an optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers are input to the closed-loop controller and the closed-loop controller stabilizes the absolute wavelength and optical bandwidth of said tunable laser radiation; an OCT interferometer; and an OCT detector; wherein at least a portion of the said tunable laser radiation is directed to the OCT interferometer and the output of the OCT interferometer directed to an OCT detector for generating OCT interferograms.
 45. The swept source optical coherence tomography system of claim 44, wherein said tunable semiconductor laser is a tunable MEMS-VCSEL.
 46. The swept source optical coherence tomography system of claim 44, wherein said closed-loop controller implements a proportional-integral-derivative (PID) algorithm based on said timing information.
 47. The swept source optical tomography system of claim 44, wherein said timing information is comprised of a timer starting point generated by said at least one reference wavelength filter and a second timing marker generated by said at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
 48. The swept source optical tomography system of claim 47, wherein said second timing marker is generated by selecting the appropriate ‘Nth’ pulse from said optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
 49. The swept source optical tomography system of claim 44, wherein said at last one reference wavelength filter and said at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises a fiber Bragg grating (FBG) with multiple reflection peaks.
 50. The swept source optical tomography system of claim 44, wherein said at least one reference wavelength filter and said at least one optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises a planar lightwave circuit (PLC) with integrated wavelength monitoring circuit (WMC). 